1. Field of the Invention
The present invention relates to a locator for determining the position of a sonde. Such a sonde may, for example, be mounted on a boring tool and may generate electromagnetic signals which enable the position of that boring tool to be determined by the locator.
2. Description of Related Art
In WO 96/29615 corresponding to U.S. Pat. No. 5,917,325, the disclosure of which is incorporated herein by reference, we discussed an arrangement for locating an underground boring tool in which a magnetic field generated by a solenoid was detected at two measuring locations. Use may be made of the relationship between the axial and radial fields of the solenoids, and a tilt sensor was included to indicate if the axis of the solenoid was not horizontal. The solenoid and tilt sensor may then be incorporated in a sonde. In general, the locator uses an aerial array to measure field components in three mutually perpendicular directions. Using amplitude and phase data from each aerial array, the location of the sonde, and hence the boring tool, can be determined.
Thus, as shown in FIG. 1, an underground boring tool 10 has a sonde 11 which contains a solenoid which generates a magnetic field, with the axis 12 of the solenoid being aligned with the longitudinal axis of the boring tool 10. The axis 12 will be the direction of movement of the boring tool, unless the boring tool is diverted by ground variation or obstacles, or by deliberate steering action. A locator having an aerial array comprising three coils X3 Y3 and Z3 will, when placed directly above the sonde 11 and in alignment with it so that the Y3 coil is parallel to the axis 12, the Z3 coil is perpendicular to the axis in a way which makes that coil Z3 vertical when the axis 12 is horizontal, have a maximum signal in the direction axis 12 with the X3 and Y3 coils detecting a null field. Any diversions from this position will result in signals from the X3 and Z3 coils, which signals will vary in amplitude and phase according to the extent and direction of such divergence. A similar arrangement will apply if the locator is positioned ahead of the sonde 11 although the signal magnitude will then be reduced due to distance. In tracking a boring tool, it is common to position a locator ahead of the tool in the anticipated direction of its motion.
However, if the locator is not correctly aligned, an incorrect measurement will result. Whilst the operator may endeavour to maintain the orientation of the locator relative to the boring tool 10, this is not always reliable since the boring tool cannot be seen.
A locator is provided with a detector for detecting the magnetic field generated by, for example, a solenoid in an inaccessible object which might be an item such as an underground boring tool. The detector preferably comprises an aerial array of three mutually perpendicular coils for detecting the generated magnetic field. When the locator is moved from a first position to a second position errors may arise in the measurement due to misalignment of the locator. In order to minimize such errors, a magnetic compass is provided on the locator to insure that the locator can be maintained in the same position relative to the earth""s magnetic field. The magnetic compass is not affected by the magnetic field generated by the solenoid as this magnetic field is an A.C. magnetic field.